1. Field of the Invention
This disclosure relates to non-intrusive scanning for materials (such as, for example, detection of explosives, nuclear materials, or contraband at airports, seaports, or other transportation terminals), and more particularly, to a method using nearly monochromatic and tunable photon sources with nuclear resonance fluorescence.
2. Background Information
Several factors may interfere with or limit the efficiency of the non-intrusive inspection of containers with Nuclear Resonance Fluorescence. In particular, non-intrusive inspection can be affected by phenomena that depend on the spectrum of photons used as the interrogating beam. Among others, these phenomena include: background noise that arises from photons in the beam that are non-resonant with any nuclear species of interest; background that arises from resonance fluorescence from nuclear species other than the species of interest present in the viewed voxel; radiation exposure that arises from photons in the beam that are non-resonant with nuclei of interest; and detector dead times due to non-resonant photons scattered by multiple processes. Therefore, the use of a photon beam with a limited range of photon energies can be advantageous over the use of a photon beam such as a bremsstrahlung beam. Some advantages associated with imaging with a narrower spectrum photon beam may include: increasing the speed of inspection; increasing the efficiency with which contraband is detected; decreasing the rate of false positive detection events; and/or decreasing the radiation dose delivered to the cargo volume.
The use of NRF measurements with some monochromatic energy sources has been demonstrated. For example, U.S. Pat. No. 5,040,200, Gamma-Gamma Resonance in Activation Analysis, and Particularly, its Application to Detection of Nitrogen Based Explosives in Luggage (Ettinger et al.), teaches scanning for a species of interest by using a sample of excited atoms of that species to generate photons that are resonant with nuclear transitions in that species of interest. A disadvantage of such sources is that they can only be used to scan for that single species of interest.